Concerning plastic lampholders: I've seen many abused ones which crackled, discolored and stank like old fish. Personally, I only buy metal ones, but I've to deal with the cheapest imagineable plastic ones because I'm in charge of changing bulbs in our appartment building, and the new basement lights that replaced the 1914 porcelaine ones are hardware store type IP 44 and fall apart upon touching them (just had the pleasure of dealing with two of them today, one fell apart completely). One of the porcelaine ones was broken and dangerous, but the other one would probably have lasted forever (but it was a pity to have the beautiful thing with it's white glass shade hanging in the basement, just imagine someone with a ladder passing by and *smash*..., so when we rewired I took them down and salvaged them)
Quality IP 44 fixtures here have a porcelaine socket inside a plastic and glass enclosure, not just a tiny backstabbed plastic monster. At least they supplied silicone tubing to protect the wires from heat.
